Dr. Wicklin. Thank you for your work, which I have found to be an exceptional resource. A practical question: Would you care to comment on the possibility of computing the MD or MD^2 in the presence of a covariance matrix (and also invereted covariance matrix) which is not positive definite? Some MD^2's could be negative as a result. Assuming the original covariance matrix did not become non positive definite because of sampling problems (id est, those negative values in the covariance matrix have true meaning), is it reasonable to attempt an interpretation of a negative MD^2, or the magnitude of negativity? Should just those negative MD^2 points be discounted, or are ALL results coming from a non positive definite covaraince matrix invalid to begin with?